1. Field of the Invention
The present invention relates generally to actuators, and more particularly to mechanical stepper motor like actuators. The actuators are particularly suitable for actuating control surfaces in gun-fired projectiles, mortars and missiles.
2. Prior Art
Since the introduction of 155 mm guided artillery projectiles in the 1980's, numerous methods and devices have been developed for the guidance and control of subsonic and supersonic gun launched projectiles. The majority of these devices have been developed based on missile and aircraft technologies, which are in many cases difficult or impractical to implement on gun-fired projectiles and mortars. This is particularly true in the case of actuation devices, where electric motors of various designs have dominated the guidance and control of most guided weaponry.
In almost all guided weaponry, such as rockets, actuation devices and batteries used to power the same, occupy a considerable amount of the weaponry's internal volume. In recent years, alternative methods of actuation for flight trajectory correction have been explored, some using smart (active) materials such as piezoelectric ceramics, active polymers, electrostrictive materials, magnetostrictive materials or shape memory alloys, and others using various devices developed based on microelectromechanical (MEMS) and fluidics technologies.
In general, the available smart (active) materials such as piezoelectric ceramics, electrostrictive materials and magnetostrictive materials need to increase their strain capability by at least an order of magnitude in order to become potential candidates for actuator applications for guidance and control, particularly for gun-fired munitions and mortars. In addition, even if the strain rate problems of currently available active materials are solved, their application to gun-fired projectiles and mortars will be very limited due to their very high electrical energy requirements and the volume of the required electrical and electronics gear. Shape memory alloys have good strain characteristics but their dynamic response characteristics (bandwidth) and constitutive behaviour need significant improvement before becoming a viable candidate for actuation devices in general and for munitions in particular.
The currently available and the recently developed methods and devices or those known to be under development for guidance and control of airborne vehicles such as missiles, have not been shown to be suitable for gun-fired projectiles and mortars. In fact, none have been successfully demonstrated for gun-fired guided munitions, including gun-fired and mortar rounds. This has generally been the case since almost all the available guidance and control devices and methodologies suffer from one or more of the following major shortcomings for application in gun-fired projectiles and mortars:                1. Limited control authority (generated force or torque) and high speed actuation capability (dynamic response characteristics or “bandwidth”), considering the dynamics characteristics of gun-fired projectiles and mortars.        2. Reliance on battery-based power for actuation in most available technologies and the requirement of a considerable amount of electrical power for their operation.        3. The relatively large volume requirement for the actuators, batteries and their power electronics.        4. The high cost of the existing technologies, which results in very high-cost rounds, thereby making them impractical for large-scale fielding.        5. Survivability problems of many of the existing devices at high-g firing accelerations and reliability of operation post firing, particularly at very high setback accelerations of over 60,000 Gs.        6. Relative technical complexities involved in their implementation in gun-fired projectiles and mortars.        
A need therefore exists for actuation technologies that address these restrictions in a manner that leaves sufficient volume onboard munitions for sensors, guidance and control and communications electronics and fuzing as well as the explosive payload to satisfy lethality requirements.
Such actuation devices must consider the relatively short flight duration for many of the gun-fired projectiles and mortar rounds, which leaves a very short time period within which trajectory correction has to be executed. Such actuation devices must also consider problems related to hardening components for survivability at high firing accelerations and the harsh environment of firing. Reliability is also of much concern since the rounds need to have a shelf life of up to 20 years and could generally be stored at temperatures in the range of −65 to 165 degrees F.
In addition, for years, munitions developers have struggled with placement of components, such as sensors, processors, actuation devices, communications elements and the like within a munitions housing and providing physical interconnections between these components. This task has become even more prohibitive considering the current requirements of making gun-fired munitions and mortars smarter and capable of being guided to their stationary and moving targets, therefore requiring high power consuming and relatively large electrical motors and batteries. It is, therefore, important for all guidance and control actuation devices, their electronics and power sources not to significantly add to the existing problems of integration into the limited projectile volume.